1. Field of the Invention
The present invention relates to electrical interconnection systems and more specifically to improved signal integrity in interconnection systems, particularly in high speed electrical connectors.
2. Background of the Related Art
Electrical connectors are used in many electronic systems. It is generally easier and more cost effective to manufacture a system on several printed circuit boards (“PCBs”) that are connected to one another by electrical connectors than to manufacture a system as a single assembly. A traditional arrangement for interconnecting several PCBs is to have one PCB serve as a backplane. Other PCBs, which are called daughter boards or daughter cards, are then connected to the backplane by electrical connectors.
Electronic systems have generally become smaller, faster and functionally more complex. These changes mean that the number of circuits in a given area of an electronic system along with the frequencies at which the circuits operate, have increased. Electrical connectors are needed that are electrically capable of handling more data at higher speeds.
One of the difficulties in making a high density, high speed connector is that electrical conductors in the connector can be so close that there can be electrical interference between adjacent signal conductors. As signal frequencies increase, there is a greater possibility of electrical noise being generated in the connector in forms such as reflections, crosstalk and electromagnetic radiation. Therefore, the electrical connectors are designed to limit crosstalk between different signal paths and to control the characteristic impedance of each signal path.
A conventional electrical interconnection system is shown in U.S. Pat. No. 7,581,990 to Kirk et al., which has been partly reproduced in FIGS. 1(a)-(b). The contents of the Kirk et al. patent are incorporated herein by reference. As shown in FIG. 1(a), the system includes a daughter card connector and a backplane connector. The daughter card connector has one or more connector wafers, each with electrical conductors having pin contacts at one end which connect to a PCB, dual-beam mating contacts 10, 12 at an opposite end, and an intermediate portion therebetween which connects the pin contacts to the mating contacts 10, 12. The intermediate portion is embedded in the wafer insulative housing 5, and the blades 20, 22 are embedded in the insulative housing of the backplane connector shroud. The mating contacts 10, 12 connect to the blades 20, 22 in the backplane connector.
Referring to FIGS. 1(a) and (b), the mating contacts 10, 12 include a differential pair of signal contacts 10 and ground contacts 12 on either end of the differential pair. The backplane blades have signal blades 20 and ground blades 22, which couple with the corresponding signal mating contact 10 and ground mating contacts 12, respectively. Each of the mating contacts 10, 12 include dual beams 14, 16 which have curved distal ends that couple with the backplane blades 20, 22. One advantage of this contact configuration is that the blades only need to be plated (such as with gold) on the one surface where they contact the beams and the beams do not require complex manufacturing techniques and are easier to reduce in size.
The mating contacts 10, 12 and the blades 20, 22 have a coplanar waveguide structure which guides the signals in the intermediate portion of the connector. The electrical characteristics of the daughter card and backplane conductors are controlled by the thickness of the metal (to a small extent), by the width of the signal and ground conductors 10, 12 (to a large extent), as well as by the spacing between the signal conductors 10 and the ground conductors 12, and the spacing between the two signal conductors 10 which form the differential pair. It is also influenced by the dielectric constant and the nature of the insulating materials surrounding the conductors 10, 12. It is desirable for the characteristic impedance of the signal and ground conductors 10, 12 to match the characteristic impedance of the signal and ground blades 20, 22 with which they connect. However, it can be challenging to obtain a mating interface which has a desired impedance because in the area where mating conductors 10, 12 overlap, the effective thickness of the conductors can be too great and the spacing between different conductors too narrow.
In order to ensure a reliable signal connection under actual use conditions, the blades 20, 22 must extend past the beams 14, 16 since the point of contact must slide for some distance along the blades 20, 22 to ensure that the connector is fully and reliably mated. The over-travel region of the blades 20, 22 is the portion above the point of contact at which the contacts 10, 12 mate with the blades 20, 22. The over-travel region acts like an excess capacitance at low frequencies and like a resonant stub at higher frequencies (e.g., 10 GHz and higher). In FIG. 1(b), the outside edges of each of the bifurcated beams are close together to be narrower than the respective mating blades, which will tend to raise the impedance of the beam region to partially compensate for the excess capacitance and the impedance-lowering characteristic of the over-travel region at frequencies below the first possibility of a stub resonance. However, because the distance between the outer edges of a blade is wider than the distance between the outer edges of the beams that mate with it, the over-travel portion of the blades couple together with each other more strongly than the outer edges of the bifurcated beams couple with each other.
Consequently, the prior art of FIGS. 1(a) and (b) do not reduce the problems of excess capacitance and resonant stub effect at higher frequencies where strong currents and charges appear in the over-travel region of the blade. The width and spacings between the various stub portions of signals and grounds affect the magnitude of that excess capacitance and the magnitude of the resonant stub effect. The blades 20, 22 are wider and more closely spaced to each other than the corresponding outer edges of the beam portions of the conductors 10, 12. This results in a deleterious effect on the electrical characteristics of the mating interface due to the stronger coupling between the stub portions of the blades. This, in turn, results in diminished signal transmission, increased signal reflection and crosstalk at the mating interface and results in diminished impedance matching. The stub does not form part of the intended path, but a parasitic path.